The present invention relates to a process and an apparatus for focusing a light beam, in particular a laser beam, on an object impacted by said laser beam.
The processing of workpieces by means of laser beams is gaining steadily in importance. One of the most significant problems in this context is that of focusing the laser beam on the surface to be worked in an optimum manner while continuously maintaining this optimum focus setting and constantly regulating it.
Various methods are known for focusing and automatic controlling focus setting. One of these is described for example in German Pat. No. 2,034,341 and U.S. Pat. No. 3,689,159. In the method described in these patents, the beam spot produced on the object by the working laser beam or an auxiliary laser is reproduced on a measuring diaphragm oscillating in the direction of the optical axis, with the neutral position of the measuring diaphragm coinciding with the image plane of the beam spot in the case of ideal focusing. The light passed by the diaphragm is guided to a photoreceiver. With optimum focusing, the image of the beamspot is located exactly in the center of the oscillating path of the measuring diaphragm and the variation in time of the measuring signal produced by the photoreceiver yields a symmetrical, essentially sinusoidal curve. In case of deviations from the optimum state of focusing the image of the beam spot is displaced from the center of the oscillating path of the measuring diaphragm and the measuring signal becomes correspondingly asymmetrical. This asymmetry is then evaluated by means of a phase sensitive detector for the setting and continuous regulation of focusing.
A further system described, for example, in German Pat. No. 2,453,364 uses in place of an oscillating measuring diaphragm an oscillating focusing lens.
These known systems work very well in principle, but have a number of disadvantages. They require, for example, a mechanically oscillating measuring or objective lens with all the inherent disadvantages of mechanically moving parts, and they are relatively demanding with respect to electronics. Furthermore, they are intended primarily for continuous lasers and are less suitable for pulsed lasers. Finally, they have no particular sensitivity in the vicinity of the optimum focusing point.
French Pat. No. 94,871 sescribes a laser scanning system for surface reliefs operating essentially without moving parts. In place of one moving measuring diaphragm the beam spot is reproduced simultaneously on two stationary measuring diaphragms. The two measuring diaphragms are located in front of and behind the image plane of the beam spot in the case of ideal focusing. The light passed by the measuring diaphragms impacts on two photoreceivers, the output signals of which represent a measure of the state of focusing or the dimension in depth of the relief scanned.
In this known system the light emitted by the beam spot and finally impacting the two photoreceivers is divided into two separate measuring beam paths by means of two beam splitter/mirrors or a mask in a spatially, entirely inhomogeneous manner. Mechanical and thermal effects and those caused by electrical/magnetic fields and refractory index variations, may affect the two measuring beam paths with different intensity, thereby causing significant measuring errors. The elimination or compensation of harmful effects, on the other hand, is very difficult, laborious and expensive.
U.S. Pat. No. 3,614,456 shows a further focusing system, which operates in a manner similar to that of French Pat. No. 94,871, but with a single measuring diaphragm. This system reacts very sensitively to interference, is difficult to adjust and is therefore not suitable for practical use in systems with high precision requirements.